Cheat Mode
These passwords are entered at the Extras screen under Password after beating the game once on any difficulty. The items will be made available upon loading your save game or starting a new game (in which case you will gain access to them when you meet up with Metal Gear Mk. II; iPod songs are immediately available.)

Bonus Weapons
Bandanna - Complete a single-player session without killing anyone (including bosses). You can do this on any difficulty.
Thor 45-70 - Earn the Fox Hound emblem on a single-player session.
Digital Camera - In the Nomad vehicle during the mission intermission; you can unlock a special photo by picking up this item in stage 4 instead of earlier.
Scanning Plug S - Log more than 10 hours of Metal Gear Online play time on the same profile as your Metal Gear 4 game. You purchase this from Drebin.
Solar Gun - Collect the five statues (those of the four Battle Beauties and the Frog Soldier/Haven Troopers in stage 1) using non-lethal means on their respective idols.
Desert Eagle, Long Barrel - Earn the Fox emblem on a single-player session.
Stealth Item - Complete a single-player session without instigating a single alert (caution is okay, but not alert). You can do this on any difficulty.
Patriot Future Assault Weapon - Earn the Big Boss emblem on a single-player session.
Race (Ricochet) Gun - Clear the single-player game once.
World War I Pistol - Earn the Hound emblem on a single-player session.

Inheritence
You carry over your Drebin points, weapons, ammunition, items, and special items from a previous game upon game clear. However, Snake will need to encounter Metal Gear II in the first stage to get his stuff back. Be sure to save your cleared data upon game completion to track your items. Load the cleared data to reap your benefits.

Extra Otacon's Codes
Enter these codes instead of the code Otacon tells you to at Shadow Moses. You will get the Access Denied scene but afterwards you will get the bonus.

Refill Psyche During Cut-scenes
If during a cut-scene something causes Snakes psyche gauge to decrease just press [X] rapidly to replenish it.

Beauty and the Beast Photoshoot
While fighting the second form of any B&B unit stall for 3 minutes. After that the whole area around Snake and the beast will go white. If you have obtained the camera already once you pull it out the Beast will pose for you.